Color temperature and exposure meter



Feb. l2, 1952 w. H. HARRISON 2,585,245

COLOR TEMPERATURE AND EXPOSURE METER Filed July 15. 1949 Patented Feb. 12, '1952 UNITED STATES PATENT OFFICE William H. Harrison, Angeles, Call-f.

Application July 15,

1s claims. l

My invention relates generally to photometric apparatus, and more particularly to an instrument for measuring -the color quality of light in terms oi color temperature. While previous apparatus has been designed for measuring color temperature of light, it has usually been quite complicated and in many cases has had the further disadvantage of depending for its accuracy to a large extent on human judgment. Meters of the latter type are often constructed with a large number of lters of varying density and color, and a scene to be photographed is viewed through an aperture over which the filters may 'be placed, and the filters adjusted to produce a certain effect on the human eye. The color sensitivity of the human eye varies from person to person, however, and for this reason: meters which rely on human judgment are not capable of the extreme accuracy required in color photography.

Attempts have been made to overcome the foregoing dimculty by employing photocells in place oi' the human eye in observing the color of incident light and the effect of iilters thereon, but photoelectric color meters heretofore available have been quite expensive, due to the fact that the photocells employed must be selected for a particular range of color sensitivity and the meter carefully balanced by a selection oi the proper cell and filters. A further disadvantage oi the photoelectric color meters heretofore available is the fact that they are quite complicated to operate and require a number of separate readings and adjustments before the nal result is obtained.

With the foregoing diiilculties in mind, it is a major object of the present invention to provide a photoelectric color meter which is in the form of a relatively simple attachment to be used with conventional photoelectric light meters commbnly known as exposure meters.

Another object of the invention is to provide a color temperature meter of the class described which may .be readily adjusted to suit the color sensitivity' characteristics of substantially any photoelectric cell with which it may be used.

Still another object of the invention is to provide a photometer which is extremely simple to operate and may be used by amateur photographers without the necessity of elaborate instruction and practice in its use.

A further object of the invention is to provide a color meter which may be read directly in terms of color temperature whereby it may be used directly in connection with conventional 1949, Serial No. 104,877

color iilms without the necessity of translating the readings of the meter by use of elaborate charts, graphs and calculations.

An additional object of the invention is to pro- 5 vide an exposure meter for use in color photography with which exposure data may. be obtained which are correctly related to the spectral sensitivity range of the lm being used and the light reaching the subject.

The foregoing and additional objects and advantages of the invention will be apparent from a consideration of the following detailed description thereof, the invention being embodied in an attachment for a conventional light meter.

l5 Briefly, the color temperature meter embodying the present invention involves the measurement of the relative intensity of two narrow color bands, one of such bands being in the red zone near one end oi the visible spectrum, and the other being in the blue range near the other end of the visible spectrum. Experience has shown that by knowing the relative intensity of light of two widely separated and narrow spectral bands as above described, a camera lens can be balanced by the use of lters of appropriate color and density so that the pictures taken on a particular color lm will have the optimum distribution of color sensitivity and thus produce a natural-appearing color photograph.

Most modern color lms are formulated or balanced for use with light of a particular color temperature. For example, some lms are designed for use in normal daylight, i. e., light of i approximately 5900 K. Film for use with tungsten incandescent lights is balanced for a color temperature of approximately 3200 K. Film for use with photoood light is balanced at approxmately 3400 K.

Thus, i1.' measurement of the source of light 40 illuminating a particular scene to be photographed in color reveals that the color temperature, is, let us say, 4300 K. and the film to be used is balanced for daylight, i. e., 5900 K., a blue 'filter of su'icient intensity is placed in front of the camera lens whereby to compensate forA the reddish characteristics of the light as compared with daylight. When such a filter is used, the resulting color photograph will closely approach the result that would be obtained if the scene had been illuminated with daylight at In addition to the desiderata above listed, it 1s desirable that the meter at present under discussion, and in fact exposure meters in general, be

constructed with standard printed scales and that individual hand calibration of scales of different meters be' avoided. Because of certain inherent variables in photocells now commercially available, it has been in the past necessary to individually calibrate photoelectric color exposure meters. Other variable characteristics in the elements employed in color meters further complicate the quantity production thereof. In general, the variables contended with are as follows: variations in the density and selectivity of color filters from one set to another; variations in the color selectivity characteristics-cr photocells from one to another; variations in overall output or response from one cell to another; variations in microammeter sensitivity from one to another.

An additional advantage of the present construction is the fact that it provides for a number of' dii'ferent adjustments by which it is possible to compensate for all of the above variables, or substantially any combination or permutation thereof, and reduce the deflection oi' the meter to one which can be read against a standard printed scale which is the same for all meters.

For a more detailed description of the attachment embodying the present invention, reference should now be had to the attached drawings in which:

Figure 1 is a perspective view of the color temperature attachment secured to a conventional exposure meter;

Figure 2 is an elevational section taken on the line 2-2 in Figure l, the conventional exposure meter being shown in phantom line;

Figure 3 is a horizontal section taken on the line 3-3 in Figure 2;

Figure 4 is an elevational view of a. filter assembly forming a part of the construction illustrated in Figure l;

Figure 5 is a fragmentary elevational view of a shutter assembly employed in the construction shown in Figure l:

Figure 6 is a horizontal section taken on the line 6-6 in Figure 4; and

Figure 7 is an elevational view of a modified form of one of the shutter blades employed in the construction shown in Figure 5.

In the drawings, the attachment is indicated generally by the reference character I0 and is secured to a conventional exposure meter II, the latter having a photocell I2 connected to actuate a sensitive electric current indicator I3, in the present case a microammeter, in accordance with the intensity of light striking the photocell I2. The construction of such photoelectric exposure meters being well known in the art, no further description is needed herein.

The attachment I0 includes a flared base member I5, having a rectangular tubular portion I6 adapted to snap over the forward end of the exposure meter II, and having suitable detents |I formed therein to engage projections I8 formed in the forward end of the exposure meter II4 The base member I5 is formed with forwardly projecting, vertically disposed track members 2D (see Figure 3), each of which has formed therein vertical channels 2| and 22, the purpose of which is to receive the vertically slidable filter assembly and vertically slidable shutter blades, as will hereinafter be described.

'I'he attachment I0 is enclosed with a pan-like cover 24 which has a rectangular aperture 25 formed therein, in alignment with the photocell I2. The cover 24 is held in place by screws 26 threaded into the track members 20. A diffusing screen. such for example as a plate of ground glass or translucent plastic material, may in some instances be secured to the front surface of the cover 24, covering the aperture 25, as indicated by the reference character 21. The diffusing screen 21 may be cemented to the cover 24 or secured thereto in any other suitable manner.

Positioned within the space between the base member Il and the cover 24 is a wedge-filter assembly ll, illustrated in Figure 4. The assembly 3D comprises a frame 3| having two horizontally disposed. rectangular openings therein, each of which receives a wedge-filter and mounts the same for limited sliding motion back and forth in the rectangular opening. The uppermost wedge 32 is blue and the lowermost 33 is red. While this arrangement is convenient in the present design, it will be obvious that the positions of the wedges can be reversed if desired, necessitating only a revision in the meter scale or conversion tables as will hereinafter appear.

The elements 32 and 33 are constructed as optical wedges, that is, the optical density increases along the horizontal axis thereof, the construction giving rise to this characteristic being illustrated in Figure 6. Here it will be seen that the wedge. for example 22, is constructed of two glass plates 34 having colored thermoplastic material 35 disposed therebetween and being wedged apart by a small wire 3i disposed at one end of the filter plates whereby to thicken the thermoplastic layer 35 at this end. Thus. referring again to Figure 6. it will be seen that the wedge' is relatively transparent at the left end and relatively dense at the right end. The thermoplastic material 35 serves also as an adhesive to secure the two plates 34 together,

While the wedge-filter 20 just described has been found suitable for use in the present structure, it will be realized that various other types of optical wedges constructed of solid glass or solid plastic material may be employed with equal efiicacy.

The frame 2| is formed with flanged rails, as indicated at 3l in Figure 2, so as to form horizontal guides for the wedges I2 and Il, in which the latter may slide horizontally within the frame 3|. A curved leaf spring 39, mounted within the frame 3| and having its ends bearing against the wedges 32 and 33. urges the latter to the right (as in Figure 4). The wedges 32 and 33 abut at their rightends against adjustment screws 40 and 4| so that the lateral position of the filter 32 may be adjusted by turning the adjustment screws 40 or 4I. An aperture 42, formed in the cover 24, gives access to the screws 40 and 4|, the entire frame 3| being moved upwardly or downwardly to place the desired screw 4l or 4| opposite the aperture 42.

The frame 3| is urged downwardly in the tracks 2I by a leaf spring 44, secured to the upper end of the frame 3| and anchored against the underside of the top flange of the cover 24. A downwardly projecting disc-shaped handle 45 is secured to the bottom edge of the frame 3| and projects downwardly through a slot in the cover 24, whereby the filter-carrying frame 3| may be pushed upwardLv to a position where the lowermost or red wedge 33 overlies the photocell I2, or selectively. the frame may be allowed to drop downwardly under the urging of the spring 44 so that the upper or blue wedge 32 overlies the photocell |2.

Between the filter frame 3| and the photocell I2 is a two-bladedadjustable shutter assembly, indicated generally by the reference character 50, and

illustrated in Figure 5. In general, the operation of the meter consists in comparing readings of the intensity or ilux value of two different colors of light, one taken through the red wedge and v,.- the other taken through the blue wedge. While i such a comparison could be made arithmetically from the absolute values of the two different colors, as will be described hereinafter, such comparison readings are preferably obtained by first adjusting the amount of light. reaching the cell so as to obtain a standard reference reading with one filter in front of the cell and then comparing that reading with the reading obtained with the other wedge in front of the cell. all other adjustments being unchanged. The purpose or the shutter 50 is to adjust the amount of light reaching the cell to determine the reference reading above mentioned.

The two blades 5I and 52 of the shutter assembly 50 are mounted face-to-face in the channel 22 and are independently sldable therein. Each of the blades 5I and 52 has a generally rectangular opening 53 and 54 (respectively) formed therein, the size and disposition of the openings 53 and 54 being such that when the two blades 5I and 52 are slid toward each other, the openings coincide and substantially all of the sensitive surface of the .photocell I2 is exposed. whereas when the twoblades are slid in opposite directions to the position indicated in Figure 5, the shutter is closed and the photocell I2 covered. As can be seen best in Figure 5, the lower edge of the aperture 53 and the upper edge of the aperture 54 are serriform whereby to reduce the rate of light reduction for a given shutter movement when the A blades are close together as compared to the rate when they are relatively widely spaced.

The coordinate movement of the two shutter blades 5I and 52 is accomplished by means of knurled operating disc 55 rotatably mounted on the base member I5 below the optical axis in the position indicated in Figures 2 and 5. Each of the shutter blades 5I and 52 is linked to the operating disc 55 by means of a slot 56, formed in the shutter blades, and a pin 51 projecting into the slot 56 and secured to the disc 55. As best seen in Figure 5, the operating pins 51 are diametrically opposed on the disc 55 so that when one is moving up due to the rotation of the disc. the other moves downwardly. Thus, as the disc 55 is rotated in one direction, the two blades 5I and 52 are brought together to close the aperture. whereas when the disc 55 is rotated in the opposite direction, the blades are moved apart to open the aperture. Since the blades 5I and 52 are in close sliding contact, the pin 5l! would strike the overlying blade 5I if the latter were flat over its entire area. Accordingly, the blade 5t is coined or die stamped to form an arched recess 58 disposed to clear the pin 5l. as shown in Figures 2 and 5.

From an examination of Figures 2 and 5, it will be noted that the size and disposition of the handle disc of the lter frame 3 I, and the shut ter-operating disc 55 are such that when the disc 45 is pressed to its uppermost position, the two discs are in substantial augment whereby the shutter-operating disc 55 may be rotated in one direction or the other with the same finger that is used to raise the iilter frame handle 45. Such operation is illustrated in phantom line in Figure 2, and indicated by the reference character 55.

As will be hereinafter described in more detail, it is sometimes desirable that the shutter close at one end before the other end is complete- 1y closed. I'br this purpose, a modiiled form of shutter blade. illustrated in Figure 7, is provided. Here it will be seen that an edge portion Il of a modified shutter blade 52a is secured to the shutter blade by a pivot pin 5I at one end and a semicircular slot l2 and set screw 53 at the other end. Thus. by loosening the set screw I3, the edge portion I3 may be swungbbout the pivot II whereby to slope the edge of the shutter aperture in one direction or the other as may be desired.

Having set forth the construction of the invention, the operation is as follows. Initially. the attachment I3 must be adjusted to suit the particular photocell in the exposure meter II. Such adjustment is accomplished in general by appropriate rotation of the adjustment-screws 4I and 4I to eflect lateral movement of the wedgelters 32 and 33 to an adjusted position in which` they may be locked by additional set screws (not shown). Once the attachment III has been adjusted to suit the particular photocell I2 involved, itis unnecessary to repeat this initial adjustment until it is desired to use the attachment in connection with a diierent photocell.

When using the meter in the manner to be described hereinafter, it is desirable that a certain standard colorof light be established, i. e., a color temperature which will cause no deection of the meter when the wedge-iilters are alternated in front of the cell. For practical uses, the standard color temperature is selected as approximately 3200 K. which is approximately the color temperature of light emitted by tungsten incandescent lamps. Since most of the important 'variations from the above standard will be in the direction of blue. it is desirable to position the reference reading of the meter obtained from such standard somewhere between the zero end of the scale and the center of the scale, and to make the shutter adjustment to obtain the reference reading with the red wedge in place in front of the cell. Accordingly, a light standard of approximately 3206 K. is employed and two wedgeillters are first adjusted so that no deflection of the meter from the reference point is obtained when the wedge-illters are alternated.

The initial adjustment is made in two stages by using standard light sources in which the relative intensities of the blue and red light are known, or in other words, light sources of known color temperature. With the exposure meter and attachment disposed to receive the light from. the iirst of the above-mentioned light sources (3200 KJ, the two filter-wedges are adjusted laterally until the meter reference reading is obtained with the blue'wedge in front of the photocell, as well as with the red wedge in front of the cell I2, i. e., there is no deflection when the wedges are alternated.

The adjustment just described creates the proper relationship of the wedges themselves and also compensates for the color characteristics of the photoelectric cell as well as the output of the micro-ammeter I3 in relation to the printed scale.

After having established the reference" reading as just described. the instrument is then checked against the secondstandard light source which has a diiferent but known color temperature. It may then be found that the reading of the meter, i. e., the deflection from the reference when the wedges are alternated, is either two small or too great to match the indications of the printed scale with the known color characteristics of the second light standard. This result A -q Vu may be due either to an erroneous output o! the photo cell (too high or too low) or improper sensitivity of the micro-ammeter Il. In either event. the error is corrected by moving both wedges simultaneously in the same direction. It both wedges are moved in a direction to place the least dense end over the cell, the deflection of the meter (the change in position of the needle) when the wedges are alternated will be decreased, whereas if the wedges are moved along their wedge axes so as to place the dense end over the cell, the spectral selectivity of each wedge-tilter. i. e., the spectral cut as it is sometimes called, will be increased and the result will be to increase the deflection of the needle. By appropriate simultaneous movement oi' the wedges along the horizontal axes. the ilnal phase of the initial adjustment is accomplished and the meter is adjusted to read against a predetermined printed scale. thus obviating the necessity of calibrating each scale independently for each particular meter of photocell.

As above stated, photocells often vary in their spectral sensitivity over their area, that is. for example, tlie center of the cell may be more bluesensitive than the outer edges. Thus, in the present instance, when such a cell was encountered, the opening and closing of the shutter 50 might not only change the intensity of light reaching the cell but might change the effective sensitivity thereof by restricting the area exposed, to one which has a different than average sensitivity. To minimize this eiect, the pivotal shutter edge pieces 6U may be adjusted to various angles (one piece 60 being provided on each shutter blade) so that the shutter closes at an angle across the cell rather than directly across. Thus, any change in spectral sensitivity between the edge and the center of the cell can be compensated for.

It will be noted that the transverse dimension of the wedge-illters 32 and I3 is considerably greater than that of the aperture 25 so that a considerable latitude of adjustment of the wedgeillters is provided for. Once adjusted to the correct lateral position, the eil'ectiveness of the particular Wedge-lter in front of the photocell is. of course, that of the average thickness of the thermoplastic 35 across the width of the aperture 25. If, however (as is often the case), the photocell i2 is predominately blue-sensitive, a further initial adjustment may be necessary so that the relative eil'ect of the wedge-nlters will be uniform for all openings of the shutter assembly 50. Such further adjustment is accomplished as above stated by properly sloping the shutter edge members 60 so that the relative meter readings for the two positions of the lter frame 3| are substantially correct for all openings of the shutter. This latter adjustment, like the former, need only be made once for any particular photocell.

In connection with the initial adjustments above described, it should be noted that while the wedge axis is shown as horizontal in the present embodiment, it can be made vertical, that is, so that the wedges change in intensity along their vertical axes. With vertical axis wedges, the initial adjustment is made by providing stops for the filter trarne which limits the vertical movement of the frame at such points as place the appropriate area of the lter in front of the cell.

Having made the initial adjustments above described, the actual'use of the attachment is as follows. The meter is held at or near the position o! the subject to be photographed and directed toward the camera. Thus. the illumination reaching the diilusion screen 21 is that which will 'reach the subject and also eifect the photograph. Wlth the meter in this position, the frame 3i isv moved to its uppermost position to place the red wedge over the photocell. such manipulation being by pressing upwardly on the handle Il. With the frame 3| 'in this uppermost position, the shutter operating disc Ii is then manipulated to open or close the shutter to a position where the predetermined "reference" reading is obtained on the 4meter II. Thereupon, the frame 3| is allowed to drop by removing the pressure on the handle Il, thus bringing the blue wedge in iront oi' the photocell. The reading then obtained with the blue wedge in position when compared to the predetermined reading just described, gives the color temperature of the incident light. If the predetermined reference reading is used for the red light measurement, the meter may be calibrated directly in color temperature umts, e. g., Kelvins. Knowing the color temperature of the incident light, it is then possible to readily adjust the camera by placing an appropriate filter in front of the camera lens so as to bring the color balance of the particular illm then in use into agreement with the color proportions or color temperature of the incident light.

Although the present embodiment of the invention is one in which the red wedge is used to establish the predetermined reference setting, the operation can be reversed and the blue wedge used to establish a reference setting and replaced with the red wedge to determine the relative reading.

While the presently preferred means for adjusting the reading oi the micro-ammeter to the reference point on the scale is the shutter Il, it will be realized by those skilled in the art that the same result can be achieved by introducing a variable electrical resistance between the photocell and the meter. Adjusting the value of such resistance will cause the meter reading to change in the same manner as adjustment of the opening of the shutter 5l.

An additional datum which is desirable in color photography is the correct exposure time. Buch a reading is obtained with the present attachment by opening the shutter assembly il to its full-open position (or, i! desired, to a predetermined'marked position), and with the attachment in this condition, taking two readings oi' y the incident light, one with the blue wedge in Vposition and one with the red wedge in poelvtion. The arithmetic average of these two readings is the mean average color intensity of the incident red and blue light and not the average ordinarily given by the photoelectric cells which are normally excessively blue sensitive. Prom this mean average reading, a more accurate exposure time may be calculated than is usually obtained with an unmodliled exposure meter.

While the forms of the attachment shown and described herein are fully capable of achieving the objects and providing the advantages hereinbefore stated, it will be realized that they are capable of considerable modification without departing from the spirit of the invention. Therefore, I do not wish to be restricted to the forms shown and described herein. but rather to the scope of the appended claims.

I claim:

1. In an exposure meter oi' the class having a assaut each of said wedges being colored to transmit a predetermined relatively narrow spectral band of light, said bands being relatively widely separated in the visual spectrum and the dimensions along the wedge axis of each of said wedges being greater than the width oi' said cell; a base member having means to detachably secure the same to said meter adjacent said cell and tracks to slidably support said frame with the plane thereof substantially perpendicular to rays of light reaching said cell and for sliding movement oi said frame in said plane between limits in which one or the other of said wedges overlies said cell and whereby said wedges may be independently moved along said rails to vary the average density of said wedge overlying said cell; a shutter disposed between said frame and cell, said shutter including a pair of blades disposed with their adjacent edges substantially perpendicular to said frame movement and mounted in said. base for selective movement toward or away from each other whereby to reduce or increase respectively, the area oi said cell exposed to light passing through said overlying wedge; an operating handle secured to said frame and adapted for digital pressure to move said frame to one limit of said movement thereof; and a rotatable operating member linked to said shutter blades to effect concurrent convergent or divergent movement of said blades when said member is rotated, said operating member being disposed behind the plane of said frame and in alignment with said handle thereof when said frame is in said last mentioned limit position whereby said operating member is readily accessible for operation when said frame is in said last mentioned position and diillcult of access when said frame is in the other limit position.

2. The construction of claim '1 further characterized in that each oi' said shutter blades is formed with a pivotable edge portion whereby to -adjust the angle between the edge of said blade and the direction of movement thereof.

3. The construction of claim 1 further characterized in that said optical wedges each comprise a pair of flat, non-parallel transparent plates in substantial contact at one end thereof, said plates being bonded together by a colored transparent thermoplastic material.

4. The construction of claim 1 further characterized by having yielding means disposed between said base and frame to urge the latter toward the limit position thereof opposite that in which said handle and operating member are aligned.

i0 to transmit a different one of two spectral bands adjacent opposite ends oi' ,a predetermined spectral range, said wedges being carried by said frame for concurrent movement therewith, each of said wedges being positioned to overlie said cell and intercept all light reaching the same when said frame is in one of said alternate positions, and each of said wedges being independently movable with respect to said frame whereby to adjust the average color density of the wedge.

overlying said cell; and means including a manually adjustable element operatively connected to said exposure meter to adjust to a predetermined reference value the reading of said indicator resulting from a given light iiux reaching said cell through one oi' said overlying wedges whereby the reading oi' said indicator with the other of said wedges over said cell is directly proportional to the color temperature of said given light.

6. In an exposure meter of the class having a photoelectric cell connected to actuate an electric current indicator, an attachment for making 5. In an exposure meter of the class having a photoelectric cell connected to actuate an electric current indicator. an attachment for making color temperature measurements comprising: a frame supported on said meter adjacent said cell for selective movement to either of two alternate positions; a pair of optical wedges,

color temperature measurements comprising: a frame supported on said meter adjacent said cell for selective movement to either of two alternate positions; a pair of differently colored optical wedges. each colored to transmit a different one of two spectral bands adjacent opposite ends of a predetermined spectral range, said wedges being carried by said frame for movement therewith, each of said wedges being positioned to overlie said cell and intercept all light reaching the same when said frame is in one oi' said alternate positions, and each of said wedges being independently movable with respect to said frame whereby to adjust the average color density of the wedge overlying said cell; and shutter means positioned in front oi' said cell and adapted to vary the total light flux reaching said cell through said overlying wedge.

'7. In an exposure meter of the class having a photoelectric cell connected to actuate an electric current indicator, an attachment for making color temperature measurements comprising: a frame supported on said meter adjacent said cell for selective movement to either of two alternate positions a pair of differently colored optical wedges, each colored to transmit a diiierent one of two spectral bands adjacent opposite ends of a predetermined spectral range, said wedges being carried by said frame for concurrent move- 'l ment therewith, each of said wedges being positioned to overlie said cell and intercept all light reaching the same when said frame is in one oi' said alternate positions, and each of said wedges being independently movable with respect to said frame whereby to adjust the average color density of the wedge overlying said cell; and electrical resistance means interposed between said cell and current indicator adapted to adjust to a predetermined value, the reading of said current indicator resulting from a given light ilux impinging on said cell.

8. In a color temperature meter, the combination of a photocell having a light receiving aperture oi' given area and a known relative color sensitivity; an electric current indicator connected to receive the electrical output of said cell whereby to indicate the light iiux impinging thereon; and a pair of optical color wedges, each having an area greater than said aperture, said wedges being mounted for concurrent movement to alternate limit positions wherein a portion of one or each colored the other o:t`A said wedges overlies said cell and inassaut tercepts all light rays impinging thereon, said wedges each being colored to transmit a predetermined spectral band ot light, said bands being adjacent the opposite ends of the visual spectrum and said wedges in addition to being concurrently movable, being independently and relatively movable to vary the average color density oi the wedge portion overlying said cell so that with a light of known selected standard color temperature intermediate said bands impinging through either of said wedges onto said cell, the indicated light flux is the same irrespective of which wedge overlies said cell whereby the color temperature of measured light reaching said cell is determined relative to said standard color temperature by the deilection oi' said current indicator when said wedges are alternated in front of said cell to alternately intercept said measured light.

9. In a color temperature meter, the combination of: a photocell having a known relative color sensitivity; an electric current indicator connected to receive the electrical output of said cell whereby to indicate the light ilux impinging thereon; a pair of optical color wedges mounted for concurrent movement to alternate limit positions wherein one or the other of said wedges overlies said cell and intercepts all light rays impinging thereon, said wedges each being colored to transmit a predetermined yspectral band of light, said bands being adjacent the opposite ends of visual spectrum and said wedges in addition to being concurrently movable, being relatively movable to vary the average color density of the wedge overlying said cell so that with a light of known selected standard color temperature intermediate said bands impinging through either of said wedges onto said cell, the indicatedlight flux is the same irrespective of which wedge overlies said cell whereby the color temperature of measured light reaching said cell is determined relative to said standard color temperature by the deiiection of said current indicator when said wedges are alternated in front oi.' said cell to alternately intercept said measured light; and shutter means secured in xed relation to said cell and overlying the same, said shutter means being adjustable vary the total light reaching said cell through said overlying wedge.

l0. In a color temperature meter, the combination of: a photocell having a known relative color sensitivity; an electric current indicator connected to receive the electrical output of said cell whereby to indicate the Vlight ilux impinging thereon; a pair oi light transmitting elements mounted for concurrent movement to alternate positions wherein one or the other of said elements overlies said cell and intercepts all light rays impinging on the same, said elements each being colored to transmit a predetermined spectral band of light, said bands being separated in the visual spectrum and adjacent the opposite ends thereof, and said elements being adjusted in relative density so that with a light oi.' known selected standard color temperature intermediate said bands impinging on said cell, the light ilux indicated by said indicator is the same irrespective of which element overlies said cell, whereby the color temperature of measured light reaching said cell is determined relative to said standard color temperature by the deilection of said current indicator when said elements are alternated in front oi.' said cell to alternately intercept said .measured light; and shutter means secured in iixed relation to said cell and overlying the same, said shutter means being adjustable to vary the el" Y' total light reaching said cell through said overlying element, and including at least one blade movable across the area of said cell, said blade having an angularly adjustable edge portion whereby to adjust the order in which predetermined areas oi' said cell are obscured as said blade is moved across the same.

l1. A color temperature meter comprising in combination: a photocell having a light receiving aperture of predetermined area; an indicator connected to said photocell to indicate the electrical output thereof; a support movably mounted adjacent said photocell for movement from one alternate position to another; a pair of light transmitting members carried by said support, each having an area substantially greater than said aperture area and so positioned on said support that in one of said alternate positions a portion oi' a ilrst of said elements overlies said aperture and in the other of said positions the other of said elements overlies said aperture, each of said elements being colored to transmit predominantly a different substantially monochromatic spectral band, said bands being at opposite ends oi' a predetermined color temperature range and at least the ilrst of said elements having progressively increasing color density along a given direction; and means to move said rst element independently in said direction with respect to said support to place a diilerent portion of said element over said aperture whereby to vary the degree ci' monochromatism of light reaching said photocell when said nrst element is positioned to overlie said photocell.

l2. The construction of claim 11 further characterized by having an adjustable shutter mounted in front of said aperture to vary the total light reaching the same through said overlying element.

13. The construction of claim 12 further characterized by having a member for manually adjusting said shutter, said adjusting member being positioned adjacent the path of movement of said support whereby to be readily accessible only when said support is in a given one of said alternate positions.

14. In an exposure meter o! the class having a photoelectric cell connected to actuate an electric current indicator, an attachment for making color temperature measurements comprising: iilter means mounted adjacent said cell for movement to either oi.' two alternate positions; a pair of differently colored light transmitting elements carried in said filter means and positioned therein to alternately overlie said cell when said illter means is moved as aforesaid, the colors transmitted by said elements being at opposite ends of a given spectral range, and at least one o! said elements being of progressively greater color density along a given direction whereby to transmit light oi' relatively greater monochromatic purity at one portion than another, said last-named element being of greater eiiective area than said cell; and means to move said lastnamed element independently with respect to said nlter means in said direction to place dii'- fering portions of said last-named element over said cell to vary the monochromatic purity of light reaching said cell through said last-named element.

15. The construction of claim 14 further characterized by having obscuring means mounted adjacent said cell and independent of said illter means to adjustably vary the amount of light reaching said cell.

assaut 16. In combination with an exposure meter of the class having a photoelectric cell connected to actuate an electric current indicator, an attachment for making color temperature measurements comprising: a frame supported on said meter adjacent said cell for selective movement to either of two alternate positions; a pair of differently colored optical wedges, each colored to transmit a different one of two spectral bands adjacent opposite ends of a predetermined spectral range, said wedges being carried by said frame for movement therewith, each of said wedges being positioned to overlie said cell and intercept all light reaching the same when said frame is in one of said alternate positions, and each of said wedges being independently movable with respect to said frame whereby to adjust the average color density of the wedge overlying said cell; and shutter means positioned in front of said cell and adapted to vary the total light 20 14 flux reaching said cell through said overlying wedge, said shutter means including a calibrated operating movement whereby said light flux may be adjusted to a predetermined value.

WILLIAM H. HARRISON.

REFERENCES CITED 'lIhe following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,898,219 Sharp Feb. 21, 1933 2,050,608 Hellige Aug. 11, 1936 2,462,823 Woodward Feb. 22, 1949 FOREIGN PATENTS Number Country Date 600,440 Great Britain Apr. 8. 1948 

